1. Field of the Invention
The present invention relates to a filter and a multiplexer for use in the microwave or millimeter-wave band, and also to a communication apparatus provided with the above-described filter and multiplexer.
2. Description of the Related Art
In a conventional dielectric resonator device, a planar-circuit-type dielectric resonator is coupled to an input/output portion, such as a coupling probe, a suspended line, or an NRD guide, as disclosed in Japanese Unexamined Patent Application Publication No. 11-234008. In the above-described dielectric resonator device, an electrode provided with an opening is disposed on each surface of a dielectric plate, and the opposing electrode openings serve as resonators. Accordingly, the dielectric resonator device may be used as, for example, a compact band-pass filter with a small insertion loss.
The use of an input/output line, such as a microstrip line or a suspended line, as an excitation line of the above-described resonator will be referred to as xe2x80x9cindirect coupling.xe2x80x9d With indirect coupling, a current flows in the input/output line due to magnetic coupling caused by excitation. This incurs a conduction loss, thereby increasing the insertion loss when the dielectric resonator device is used as a filter. Additionally, when the above-described dielectric resonator device is connected to a module using a waveguide as an input/output portion, which is frequently used in a millimeter-wave device, it is necessary to swap the waveguide with the microstrip line or the suspended line, which causes a poor connection.
Accordingly, the present invention provides a filter and a multiplexer, and a communication apparatus provided with the filter and/or the multiplexer, without a conduction loss caused by input/output lines or a poor connection with a transmission line, such as a waveguide.
According to one aspect of the present invention, there is provided a filter including a resonator having electrodes on both surfaces of a dielectric plate, the electrodes being provided with electrode openings which face each other via the dielectric plate. In the filter, a waveguide is directly coupled to the resonator, that is, the waveguide is coupled to the resonator without using an input/output line such as a microstrip line or a suspended line as an excitation line.
By directly coupling the resonator formed on the dielectric plate and the waveguide, a conduction loss caused by an input/output line, such as a microstrip line or a suspended line, can be prevented. The connection of the resonator with a module using the waveguide as an input/output portion can also be enhanced.
In the above-described filter, at least one end of the resonator coupled to the waveguide may be opened. With this structure, the magnetic field leakage of the resonator can be increased, thereby facilitating a strong coupling force with the waveguide.
The width of the opened end may be set so that a desired external Q is obtained. For example, the electrode opening at the opened end may partially be narrowed. With this structure, the level of a coupling force with an external source, i.e., the waveguide, can be easily determined.
In the aforementioned filter, electromagnetic waves may propagate in the waveguide perpendicularly to the surface of the dielectric plate. With this arrangement, the connection and the mountability of the mounting portion of the dielectric plate, which forms the filter, to the waveguide can be improved.
In the filter in which the electromagnetic waves propagate in the waveguide perpendicularly to the surface of the dielectric plate on which the electrode openings are formed, the electrode opening may be extended into the waveguide by up to an amount corresponding to the resonant wavelength of the resonator, that is, by up to the maximum length from one end to the other end of the electrode opening in the longitudinal direction of the filter. With this configuration, a predetermined external Q (Qe) can be obtained.
In the above-configured filter, a length of the resonator directly coupled to the waveguide is preferably (2nxe2x88x921)/4 or 2n/4 of a resonant wavelength (where n is an integer of one or more). In such a case, the electrode opening is preferably inserted into the waveguide by an amount corresponding to (mxe2x88x921)/2 (where m ranges from 2 to n) of the resonant wavelength in the longitudinal direction of the electrode opening. With this arrangement, the coupling strength between the resonator and the waveguide can be increased, and a change in the external Qe relative to the back short can be decreased, thereby stabilizing the filter characteristics.
According to another aspect of the present invention, there is provided a multiplexer including a plurality of the filters having any of the above-described configurations.
According to still another aspect of the present invention, there is provided a communication apparatus including the aforementioned filter or multiplexer.